EARLY ONTOGENETIC PHENOMENA IN MAMMALS. 3 
Cap. PAGE 
6. Chiroptera, Carnivora, Rodentia . : x. 125 
c. Primates : ; ‘ am 27 
2. The Classificatory Value of Placentation : 72129 
3. The Phylogeny of the Placenta : : 5 Lal 
4, Summary of Chapters LV and V , : . 148 
VI. ReEFLEXIONS ON THE PHYLOGENY AND THE SYSTEMATIC ARRANGE- 
MENT OF VERTEBRATES é : : . +149 
CHapter I. THe EHaruiest CELL-LAYERS. 
INTRODUCTORY. 
The phenomenon of fecundation of the egg imaugurates 
the well-known series of cell-divisions which give rise in 
Amphioxus to a grouping of the first cleavage-cells into a 
hollow mulberry shape, whereas in cartilaginous fishes, in 
reptiles, and in birds the cleavage-cells are disposed in disc- 
shape at one point of the yolk, which latter, though origin- 
ally part of the egg, will soon take the aspect of an appen- 
dage to the embryo. Again, in Amphibia and in certain 
more archaic fishes the yolk is much less considerably deve- 
loped and the whole egg thus segmented in toto, whereas in 
the Teleosts there is an abundance of food-yolk, but a dispo- 
sition of the parts somewhat different from what we find in 
cartilaginous fishes and in Sauropsids. 
In Mammals again the whole of the egg-substance is seg- 
mented (holoblastic cleavage as against the meroblastic 
cleavage of the cartilaginous fishes and the Sauropsida), but 
the further development more and more resembles that of 
the reptiles in which a very considerable yolk is present, a 
fact that has given rise to the erroneous conclusion that the 
mammalian blastocyst was derived from the Sauropsidan by 
a process consisting in the gradual disappearance of the 
yolk, with retention of the other developmental characters. 
We will find occasion later on to discuss the value of this 
phylogenetic speculation. 
